1. Field of the Invention
The present invention relates to a suspension system for a tractor or another work vehicle.
2. Description of the Related Art
Among tractors, which are an example of a work vehicle, there are those which comprise a suspension mechanism in the front wheels, such as is disclosed in U.S. Pat. No. 6,145,859, for example. A suspension mechanism generally acts in rising and falling motions in accordance with unevenness in the ground surface, whereby the unevenness in the ground is absorbed and the ride is made more comfortable.
Worked ground on which the work vehicle travels has much unevenness in comparison with paved roads, and the work vehicle is often equipped with an implement, which causes the suspension stroke position of the suspension mechanism to deviate upward or downward of the vehicle body from a preset target range (when the suspension stroke position of the suspension mechanism is positioned in the target range, the vehicle height is maintained within the height range), whereby the vehicle height goes beyond the predetermined height range (unusual fluctuation).
In this case, it is preferable to minimize unusual fluctuations in the vehicle height of the work vehicle and to maintain the vehicle height within the predetermined range. Therefore, it has been proposed that when the suspension stroke position of the suspension mechanism deviates upward or downward of the vehicle body from the target range as previously described, the vehicle height should be maintained as much as possible within the predetermined height range by a suspension reference position variation mechanism for moving the suspension stroke position of the suspension mechanism to the target range.
A suspension system of a work vehicle such as the one previously described has the following problem in cases in which a suspension reference position variation mechanism is provided for varying the reference position of the suspension stroke in relation to the suspension mechanism. Specifically, every time the suspension mechanism operates and the suspension stroke position of the suspension mechanism deviates upward or downward of the vehicle body from the target range, the operative frequency of the suspension reference position variation mechanism is extremely high when the suspension reference position variation mechanism operates, and the suspension reference position variation mechanism has a disadvantage in terms of durability.